The present invention relates to a precursor fiber bundle to be processed into a carbon fiber bundle, a process for producing the precursor fiber bundle, a carbon fiber bundle, and a process for producing the carbon fiber bundle. In more detail, the present invention relates to a precursor fiber bundle to be processed into a carbon fiber bundle, which is low in production cost, excellent in productivity, and which experiences less fiber breakage and fuzz generation, and which can be transformed into a sub-tow having an optimum formation for supplying to a process for producing a carbon fiber bundle. This invention also relates to a process for producing the precursor fiber bundle, to a carbon fiber bundle prepared from the sub-tow, and to a process for producing the carbon fiber bundle.
Furthermore, the present invention relates to a precursor fiber bundle comprising an acrylic polymer processed into a carbon fiber bundle, a process for producing the same, a carbon fiber bundle obtained from the precursor fiber bundle, and a process for producing the carbon fiber bundle.
Conventional precursor fiber bundle to be processed into a carbon fiber bundle is made of an acrylic polymer. The fiber bundle filaments may number from 3,000 to 20,000, and have a fineness of from 1,000 denier to 24,000 denier with small occurrences of fiber breakage and fuzz. It has been used for production of carbon fiber bundles having high strength and high modulus.
The precursor fiber bundle comprising an acrylic polymer processed into a carbon fiber bundle have been widely used as reinforcing fibers for components in the field of aerospace, sports, etc. The conventional carbon fiber bundle has been mainly examined to enhance its strength and the elastic modulus of carbon fibers. Specific items of examination include degree of crystalline orientation and densifying property of the precursor fibers, single filament breakage, fuzz, adhesion between filaments, acceleration of stabilization of the precursor fibers, etc.
The utilization of carbon fibers is being expanded at a rapid pace into general industrial fields including automobiles, civil engineering, architecture, energy, compounds, etc., and it is advantageous to supply a raw fiber bundle (precursor fiber bundle) to be processed into a carbon fiber bundle as a multifilament having improved strength and elastic modulus, at lower cost, and with increased productivity.
However, the raw fiber bundle (precursor fiber bundle) intended to be processed into a carbon fiber bundle is actually produced as a multifilament and wound on a drum or bobbin, and supplied in this style to a process for producing a carbon fiber bundle. Due to restrictions in the process of producing the carbon fiber bundle, particularly restriction of thickness (fineness) of the precursor fiber bundle in the stabilizing process, the rate of productivity has been kept remarkably low.
That is, the precursor fiber bundle comprising an acrylic polymer, processed into a carbon fiber bundle, is heated in an oxidizing atmosphere having a temperature of from 200xc2x0 C. to 350xc2x0 C. for stabilizing prior to carbonizing treatment. The stabilization treatment causes oxidization and cyclization, but since it generates heat, the heat stored in the fiber bundle becomes an important factor. If the heat stored in the fiber bundle is excessive, fiber breakage and adhesion between filaments occur. So, the stored heat must be kept low enough to prevent this.
Accordingly, a precursor fiber bundle having excessive thickness cannot be supplied into the stabilizing furnace. In industrial production the precursor fiber bundle is accordingly restricted in thickness (fineness). The restriction unfortunately keeps productivity low and is an obstacle in reducing production cost.
Producing a thermoplastic synthetic fiber bundle as a raw fiber bundle to be processed into a spun yarn or a non-woven fabric, not as a precursor fiber bundle to be processed into a carbon fiber bundle, is disclosed in Japanese Patent Laid-Open (Kokai) No. 56-4724. In this process, a tow running into a crimping apparatus is divided by dividing pins located close to the entrance of the crimping apparatus. A plurality of divided sub-tows are simultaneously supplied into the crimping apparatus, so that the plurality of sub-tows may be crimped as a whole, to be collected as one crimped tow capable of being potentially divided into crimped sub-tows later. However, if this process is applied to production of a precursor fiber bundle intended to be processed into a carbon fiber bundle, fiber breakage occurs often. This lowers the grade of the product since it is necessary to divide into a plurality of sub-tows a precursor fiber bundle having a fineness of not less than 300,000 denier in which filaments are engaged with each other by mutual oblique crossing and are closed up each other. This also adversely affects the production of carbon fibers.
An object of the present invention is to provide a precursor fiber bundle that can effectively and efficiently to be processed into a carbon fiber bundle which can be larger in thickness, i.e., in fineness to provide high productivity and low production cost, and which can be easily divided into sub-tows, each of which has a thickness (fineness) as required for producing a carbon fiber bundle, considering the restriction of thickness (fineness) of the fiber bundle in the process.
A further object of the present invention is to provide a process for producing the precursor fiber bundle, and the resulting carbon fiber bundle, and a process for producing the carbon fiber bundle. Hereinafter in this specification, the expression xe2x80x9cprecursor fiber bundlexe2x80x9d means a precursor fiber bundle adapted to be processed into a carbon fiber bundle or a precursor fiber bundle for production of a carbon fiber bundle.
The precursor fiber bundle of the present invention can be kept in the form of one single tow when packed in a container, and can potentially be divided into a plurality of sub-tows when taken out of its container and used for producing a carbon fiber bundle.
The precursor fiber bundle of the present invention is an acrylic polymer fiber tow having the total fineness of about 300,000 denier to 1,500,000 denier, and preferably having a number of filaments of from about 50,000 to about 1,000,000, which can be potentially divided into sub-tows each of which has a fineness of from about 50,000 denier to about 250,000 denier.
The precursor fiber bundle may also be a crimped tow or a non-crimped tow. In the case of a non-crimped tow, its moisture content is preferably in the range of from about 10% to about 50%.
Furthermore, the degree of entanglement of each of the sub-tows divided from the precursor fiber bundle is preferably in the range of from about 10 mxe2x88x921 to about 40 mxe2x88x921, measured according to the well-known hook drop testing method. Where the degrees of entanglement are in that range, the precursor fiber bundle e.g. the original tow can be easily divided into a plurality, each of which is used for producing a useful carbon fiber bundle.
The process for producing a precursor fiber bundle having the above properties comprises the steps of dividing a fiber bundle consisting of a plurality of spun filaments into a plurality of sub-tows in such a way that each sub-tow comprises a predetermined number of filaments; drawing the filaments while in this state of division; collecting the plurality of drawn sub-tows into one tow potentially capable of being divided into a plurality of sub-tows when used for producing a carbon fiber bundle; and packing the product into a container. In this process, a plurality of groups each of which consist of a plurality of sub-tows may also be arranged to run in parallel each other.
The process for producing a carbon fiber bundle according to the present invention may also comprise the steps of dividing the precursor fiber bundle into a plurality of sub-tows; and subjecting the sub-tows to a stabilizing process and to a carbonizing process.
According to the present invention, the filaments taken up from a spinnerette are divided into a plurality of sub-tows, and the respective sub-tows are then collected into a single tow that is capable of being potentially divided into a plurality of sub-tows when used later for producing a carbon fiber bundle, and before they are packed into a container.
The precursor fiber bundle formed as a single tow is packed into a container, since the tow production speed is greatly different than the treatment speed of the subsequent carbonizing process. In the carbon fiber production process, the precursor fiber bundle formed as a single tow is taken out of the container and fed to a stabilizing process. In this case, it is divided into a plurality of sub-tows each of which has a predetermined thickness, before it is fed to the stabilizing process. Therefore, the problem of excessively stored heat, as described before, can be prevented from occurring, and carbon fibers that have the desired high strength and high modulus can be produced efficiently. In the final stage of the process for producing the precursor fiber bundle, the filaments are formed as one fiber bundle having a large total fineness, but the carbon fiber bundle after it has been produced is divided into a plurality of sub-tows each of which has a fineness suitable for stabilizing and carbonizing. Accordingly, the production of the precursor fiber bundle, and the production of the carbon fiber bundle can be carried out under remarkably efficient conditions.
The precursor fiber bundle of the present invention is preferably made of an acrylic polymer containing acrylonitrile, one or more unsaturated monomers selected from the following group A, and one or more unsaturated monomers selected from the following group B. They are present in amounts shown in the following equations (1), (2) and (3).
Group A comprises one or more unsaturated monomers selected from the group consisting of vinyl acetate, methyl acrylate, methyl methacrylate and styrene.
Group B comprises one or more unsaturated monomers selected from a group consisting of itaconic acid and acrylic acid.
The amounts are:
AN (wt %)xe2x89xa786xe2x80x83xe2x80x83(1)
3xe2x89xa6A (wt %)xe2x89xa610xe2x80x83xe2x80x83(2)
0.25Axe2x88x920.5xe2x89xa6B (wt %)xe2x89xa60.43Axe2x88x920.29xe2x80x83xe2x80x83(3)
The symbols in the above formulae stand for the following:
AN represent the acrylonitrile content (wt %) in the acrylic polymer.
A represent the content (wt %) of the unsaturated monomer selected from said group A in the acrylic polymer (total weight of unsaturated monomers when a plurality of unsaturated monomers are present)
B represent the content (wt %) of the unsaturated monomer selected from said group B in the acrylic polymer (total weight of unsaturated monomers when a plurality of unsaturated monomers are present)
As shown by the formula (2), the weight percent (content) of the unsaturated monomer selected from said group A is in the range of from about 3 wt % to about 10 wt %. If the amount is less than about 3 wt %, the filaments are slightly less likely to stretch when drawn, and the tension in the stabilizing process is too high. If said amount is more than about 10 wt %, more filaments adhere to each other when stabilized, and carbonization at a lower temperature at a lower speed is required to prevent it. This raises production cost.
Furthermore, as shown in the formula (3), the weight percent B of the unsaturated monomer B is in the range of about (0.25xc3x97Axe2x88x920.5) wt % to about (0.43xc3x97Axe2x88x920.29) wt %. If the amount is less than the lower limit, acceleration of stabilization does not occur. If the amount is more than the upper limit, acceleration of stabilization becomes less efficient; this raises production cost.
The acrylic polymer may be produced by any known polymerization method such as suspension polymerization, solution polymerization or emulsion polymerization, etc. The polymerization degree is preferably about 1.0 or more expressed as intrinsic viscosity ([xcex7]). The upper limit of intrinsic viscosity ([xcex7]) is desirably about 3.0 or less since otherwise the production of the spinning dope itself is difficult, and since otherwise the spinning stability of the polymer is also remarkably lowered. The expression xe2x80x9cintrinsic viscosityxe2x80x9d refers to the value measured at 25xc2x0 C. with dimethylformamide as the solvent.
The solution of the acrylic polymer, i.e., the spinning dope, is spun into an acrylic polymer fiber bundle using a coagulating bath of an organic solvent or water.
Spinning may be wet spinning in which a spinning dope is ejected from a spinnerette emersed in a coagulating bath, or may be semi-wet spinning in which a spinning dope is ejected from a spinnerette installed above the liquid surface of a coagulating bath with a distance between them, into air or inactive gas and introduced into the coagulating bath, or may be melt spinning.
In spinning using a solvent and plasticizer, the spun filaments may be drawn into a bath immediately, or after having been washed with water to remove the solvent and plasticizer.
The acrylic polymer fiber bundle obtained by any of these methods is drawn with a draw ratio in the range of from about 2 times to about 8 times in a drawing bath having a temperature of from about 50xc2x0 C. to about 98xc2x0 C. If the drawing ratio is too low, good densifying cannot be obtained, leaving voids, and the physical properties are likely to be poor. If the draw ratio is more than about 8 times, the tension during carbonization increases, requiring a larger apparatus. Drawing in a steam tube may be used with drawing in a bath, but in the case of drawing in a steam tube, it is preferable to keep the drawing ratio low to suppress orientation of fibers. However, drawing in a bath only is preferable.
Turning now to the number of filaments of the acrylic polymer fiber bundle, it is preferable to use a multifilament comprising a number of filaments in the range of from about 5xc3x97104 filaments to about 1xc3x97106 filaments to enhance production efficiency and cost reduction.
Subsequently, the filaments are dried under gentle air flow having a temperature in the range of from about 110xc2x0 C. to about 180xc2x0 C. or a heating roller under tension or relaxation, and are densified simultaneously. Prior to the drying and densifying, it is desirable to apply a proper oiling treatment to prevent adhesion between filaments and to facilitate handling of the dried and densified fiber bundle.
The dried and densified fiber bundle is shrunken at a ratio of about 5% to about 18%. The shrinking treatment is intended to shrink the filaments under proper tension using a heating roller or any other heating means such as hot air, and this is effective to decrease the tension acting on the fiber bundle in the subsequent stabilizing process. For decreasing tension, a shrink treatment having a ratio of about 5% to about 18% is important. The heating temperature is in the range of about 80xc2x0 C. to about 120xc2x0 C., and it is preferable to maintain substantially no tension, but some tension may be applied for the convenience of process if it allows enough shrinkage to be achieved. The percentage of shrinkage may be controlled by combining the heat treatment temperature, the residence time and the tension. The fineness (d) of each of the filaments finally obtained is preferably in the range of about 1 denier to about 2.0 deniers, more preferably from about 1.0 denier to about 1.5 deniers, for higher productivity.
The precursor fiber bundle obtained as described above may be processed into a carbon fiber bundle by any conventional method. The stabilizing conditions in this case may be as in conventional methods. The fiber bundle is treated in an oxidizing atmosphere having a temperature in the range of about 200xc2x0 C. to about 300xc2x0 C. under tension or while being drawn.
The shrinkage stress during stabilization of the acrylic polymer fiber bundle is related to the potential physical properties of the resulting carbon fiber bundle. When the raw fibers are higher in strength, that is, more highly oriented with greater shrinkage stress, the potential physical properties of the carbon fibers obtained are greater. However, in order to obtain such physical properties, it is desirable to control the shrinkage of fibers or to apply high tension to the fibers by drawing.
To obtain the physical properties of reinforcing carbon fibers for general industrial applications, high tension treatment is not required so much, and the problem in commodity design is to produce carbon fibers with good cost performance which can compete in price with conventional materials such as glass fibers, iron and aluminum.
Conventionally, carbon fibers having great tensile strength are generally produced by stabilizing precursor fibers with a high capability of shrinkage stress at a high tension, to produce, as an intermediate product, oxidized fibers (stabilized fibers) having a high degree of crystalline orientation and a high tensile strength. In such a high tension process, the occurrences of fuzz and breakage of fibers are likely to reduce quality and processability. The production conditions and equipment conditions are accordingly varied in an effort to prevent this. However, such approaches tend to raise the production cost of carbon fibers significantly.
On the contrary, according to the present invention, styrene, methyl acrylate or methyl methacrylate as a polymerizable unsaturated monomer is added to the acrylic polymer fibers, thereby achieving reduced shrinkage stress, thereby allowing the tension in the stabilizing process also to be reduced. The tension in the stabilizing process can be kept low, thus minimizing the occurrences of fiber breakage and fuzz in the stabilizing process.
Furthermore, a carbon fiber bundle of about 25,000 deniers or more in fineness, substantially having no twist, and of from about 10 mxe2x88x921 to about 100 mxe2x88x921 in the degree of entanglement measured according to the hook drop test can be obtained. Its physical properties are in the range of from about 2.0 GPa to about 5.0 GPa, preferably from about 3.0 GPa to about 4.5 GPa in tensile strength and in the range of from about 200 GPa to about 300 GPa in elastic modulus. These carbon fibers may be used for general purpose. Herein, the expression xe2x80x9csubstantially no twistxe2x80x9d means the twist count per meter is not more than 1 turn of twist.
It is preferable that the tension T in the stabilizing process approximately satisfies the following formula (4).
30xe2x89xa6T (mg/d)xe2x89xa6120xe2x80x83xe2x80x83(4)
More preferably, the tension T is in the range of from about 60 mg/d to about 100 mg/d. If the tension T is less than about 30 mg/d, the tension is so low as to shrink the fibers, and to lower the degree of crystallite orientation, and the fibers obtained are low in tensile strength. If the tension T is more than about 120 mg/d, good physical properties can be obtained, but since the tension is so high, the return rollers must be especially strong or of large diameter. The equipment must be so heavy as to be industrially undesirable. If return rollers that are large in diameter are installed for the stabilizing furnace, it is difficult to achieve a high frequency return, making mass processing difficult. Also in view of this, it is not desirable to keep the tension excessive.
In the present invention, since the tension T in the stabilizing process is controlled to low range of about 30 mg/d to about 120 mg/d, the load per unit filaments acting on the rollers is light, and unprecedented consistent carbon fiber production allows very favorable mass processing. Therefore, no equipment of excessive size is necessary; general purpose carbon fibers can be produced using inexpensive equipment, and very advantageously in view of reducing product cost. As a result, carbon fibers may now be used for applications where they could not have been used because of high cost.
The effect of cost reduction by achieving low tension is further described below.
Firstly, cost reduction can be obtained through process stability. A lower tension is effective for decreasing the creation of fuzz and fiber breakage in the strand formed as an aggregate of many short fibers during processing. Hence, the process is very effective to decrease production mishaps such as the seizure of filaments and the strand on the rollers. The amount of generated fuzz is directly related to processability. The low tension also has a good minimizing effect upon the amount of fuzz. The amount of fuzz created is a good indicator for evaluating the overall processability of the method.
Secondly, an important cost reduction can be obtained through the enhanced volume availability in the stabilizing furnace. In the carbon fiber production process, since a strand to be processed is continuously processed, a series of rollers is usually used. Since these rollers are deflected in response to the tension of the strand, a deflection which poses no problem in equipment or process stability is achieved by this invention. In the case of a cylindrical roller of uniform diameter, the maximum deflection is proportional to the product of the tension and the 4th power of (roller length L/roller diameter D). Therefore, in general, if the tension is doubled, the deflection is doubled, and to lower the doubled deflection to the original deflection, the diameter must be increased to 1.2 times. The diameter of a roller especially directly affects the volume availability of the stabilizing furnace; and if the diameter of a roller is decreased, the volume availability of the stabilizing furnace is higher, and this significantly enhances carbon fiber productivity.